Advancements in materials processing technology have caused an increasing demand for parts made from superalloys. Many high performance aircraft, for example, now employ turbine blades that are single crystals of nickel-based superalloys. The desirability of superalloys results from their high strength at high operating temperatures.
Superalloy parts are typically produced as single-crystal or directionally-solidified castings. Elaborate casting and inspection methods are employed to ensure that each part is a single-crystal or directionally-solidified crystal with a desired orientation. Even minor defects in the crystalline structure may be unacceptable as they can result in mechanical failure.
To reduce the likelihood of defects in the crystalline structure, the casting process for such parts has become a labor and time intensive process. The molten alloy is poured into a mold located in a furnace. One end of the mold is cooled to initiate crystallization. The mold is then slowly withdrawn from the furnace. The withdrawal rate is extremely slow to ensure an acceptable crystallization rate and crystal of the correct orientation. A slow crystallization rate promotes flawless crystal growth in the direction of solidification. If the rate of crystallization is too rapid, the metal will form unacceptable polycrystals and the part must be discarded. The possibility of forming parts with defects in the crystal structure with a lack of a reliable technique to otherwise monitor the rate of crystal growth during casting has to date caused engineers to select extremely conservative (i.e., slow) crystallization rates.
Further, despite the noted precautions taken during casting, defects can still occur which result in discarded parts and wasted production time. The scrapped material cannot simply be remelted and reused but must rather be retreated by an expensive refinement process before further use.